In current digital still cameras, video cameras, and so on, a bit-width (approximately 10 to 12 bits) of a signal in an input system and a processing system is set to be wider than a bit-width (normally 8 bits) of a final output signal to prevent image quality deterioration due to cancellation of significant digits during digital signal processing. In this case, tone conversion must be performed to match the bit-width of an output system.
Conventionally, tone conversion processing is performed on the basis of a fixed tone characteristic in relation to a standard scene or a tone characteristic corresponding to a main object in an image signal. Further, a method of determining a tone characteristic for each region of the image signal and performing conversion adaptively has been proposed.
With all of these methods, noise component amplification often occurs as a side effect of the tone conversion processing. Hence, JP2005-318063A proposes a method of suppressing noise amplification by setting limits on individual tone characteristics of each region when tone conversion processing is performed adaptively on each region of an image signal. Further, JP10-210323A proposes a method of improving contrast without making a noise component conspicuous by separating an image signal into a noise component and a signal component and performing tone conversion on the signal component.